Many hormones and neurotransmitters regulate body functions through specific receptors present on the cell membrane. Many of these receptors transmit signals into cells by activating conjugating guanosine triphosphate-binding proteins (G proteins). These receptors are therefore generically referred to as G protein-coupled receptors (GPCRs). Alternatively, since they also share a structure comprising a seven membrane-permeating region, they are also generically referred to as ‘seven transmembrane’ receptors.
G protein-coupled receptors are present on various functional cell surfaces in cells and organs of the body, and play extremely important roles as targets of molecules such as, for example, hormones, neurotransmitters, and physiologically active substances that regulate the functions of these cells and organs of the body. Consequently, G protein-coupled receptors have been attracting considerable attention as targets of drug development. A number of G protein-coupled receptors are known to be constitutively active (Costa, T. et al., Mol Pharmacol, 41, 549-560, 1992; Lefkowitz, R. et al., Trends Pharmaco. Sci., 14, 303-307, 1993). In some cases when a mutation is introduced into G protein-coupled receptors, their activity is known to further increase. For example, a constitutively active mutant of the a1B-adrenaline receptor, a type of G protein-coupled receptor, is known (Kjelsberg, M. A. et al., J. Biol. Chem. 267, 1430-33, 1992). Additionally, WO 01/77172 discloses constitutively active mutants of various G protein-coupled receptors.
In addition, antagonists that exhibit actions opposite to agonist have recently been discovered, indicating that the inverse agonists may be drug candidate compounds targeting G protein-coupled receptors (Milligan, G. et al., Trends Pharmaco. Sci., 16, 10-13, 1995). When inverse agonists act on G protein-coupled receptors, a conformation change arises, which is thought to increase the proportion of inactive forms (Milligan, G. et al., Trends Pharmaco. Sci., 16, 10-13, 1995).
Histamine H3 receptors (H3 receptors) are known to be a type of G protein-coupled receptor. Genes that encode these receptors are reported to exist in various living organisms, such as humans (Lovenberg, T. W. et al., Molecular Pharmacology, 55: 1101-1107, 1999; Lovenberg, T. W. et al., Journal of Pharmacology and Experimental Therapeutics, 293: 771-778, 2000; Tardivel-Lacombe, J. et al., Molecular Neuroscience, 11: 755-759, 2000; WO 2003004637). H3 receptor gene knockout mice have been found to demonstrate increased body weight, food intake and blood insulin or blood leptin levels, thus clearly indicating a correlation between H3 receptors and diseases characterized by changes in body weight, food intake and blood insulin or blood leptin levels (WO 2003004637). Furthermore, H3 receptors are constitutively active, even in their natural states, and have been reported to easily adopt constitutively active conformations (Morisset, S. et al., Nature, 408, 860-864, 2000). However, to date there have been no reports of examples of constitutively active H3 receptor mutants.